The invention relates to an engine-braking arrangement for an internal combustion engine with an exhaust-gas turbocharger including a brake valve in the exhaust pipe upstream of the turbine and a pressure relief line, which is connected to the turbine upstream of the turbine rotor and includes a control valve.
DE 195 40 060 A1 discloses a multi-cylinder internal combustion engine, in the exhaust tract of which is arranged a turbine of an exhaust-gas turbocharger, the said turbine driving, via a shaft, a compressor which generates an increased intake pressure in the intake duct. In order to achieve a high engine-braking action in the coasting mode of a vehicle, in which the engine is installed, a brake valve in the exhaust line is closed during braking, with the result that excess pressure is built up in the exhaust line upstream of the brake valve. The built-up exhaust gas flows at high velocity through a bypass directly into a spiral duct of the turbine and acts on the turbine rotor, whereupon the compressor builds up excess pressure in the intake duct. The cylinder is thereby subjected to an increased charging pressure at the entry side. On the exit side, an excess pressure develops between the cylinder outlet and the brake valve and counteracts the discharge of the air compressed in the cylinder into the exhaust tract via the exhaust valves. During braking, the piston must, in the compression stroke, perform compression work against the high excess pressure in the exhaust tract, with the result that a strong braking action is achieved.
In order to prevent an unacceptably high load caused by an excessive build-up pressure upstream of the rake valve during braking, the engine-braking device of DE 195 40 060 A1 has a discharge arrangement consisting of a relief line with a stop valve. When a pressure limit value is reached, the stop valve is opened, so that backed-up exhaust gas is discharged via the relief line and the back-up pressure is reduced.
To control the high pressure upstream of the brake valve, the exhaust-gas quantity to be discharged via the discharge arrangement must finely adjustable, since even slight pressure drops in the line section upstream of the brake valve lead to a drastic reduction in engine-braking capacity. For this reason, stringent requirements must be met as regards an accurate setting of small cross sections of the stop valve in the relief line. In particular, exhaust-gas pulsations caused by pressure fluctuations in the engine, which generate disturbing forces acting on the stop valve, should not impair the blow-off operation.
It is the object of the present invention to provide an engine-braking device with a reliably operating blow-off arrangement, by means of which overloading of the internal combustion engine during large braking loads is prevented.